We introduce the successive Hitting Set model, where the set system is not given in advance but a set generator produces the sets that contain a specific element from the universe on demand. Despite incomplete knowledge about the set system, we show that several approximation algorithms for the conventional Hitting Set problem can be adopted to perform well in this model. We describe, and exper...

We study a geometric hitting set problem involving unirectional rays and curves in the plane. We show that this problem is hard to approximate within a logarithmic factor even when the curves are convex polygonal x-monotone chains. Additionally, it is hard to approximate within a factor of 76 even when the curves are line segments with bounded slopes. Lastly, we demonstrate that the problem is ...

A monoid S satisfies Condition (A) if every locally cyclic left S-act is cyclic. This condition first arose in Isbell’s work on left perfect monoids, that is, monoids such that every left S-act has a projective cover. Isbell showed that S is left perfect if and only if every cyclic left S-act has a projective cover and Condition (A) holds. Fountain built on Isbell’s work to show that S is left ...

A monoid S satisfies Condition (A) if every locally cyclic left S-act is cyclic. This condition first arose in Isbell’s work on left perfect monoids, that is, monoids such that every left S-act has a projective cover. Isbell showed that S is left perfect if and only if every cyclic left S-act has a projective cover and Condition (A) holds. Fountain built on Isbell’s work to show that S is left ...

among 880 studied feet of 7-14 years old children 6.9% suffered mild and severe flat foot. 53.8% of the affected children were symptomatic. as 40.1% of the general population experiences symptoms, in a small proportion of affected persons, symptoms are due to flat foot. the prevalence of symptoms rises with increasing severity of the disorder. in this article, reviewing general aspects of flat ...

Erosion reduces soil productivity and causes negative downstream impacts. Erosion processes occur on areas with erodible soils and sloping terrain when high-intensity rainfall coincides with limited vegetation cover. Timing of erosion events has implications on the selection of satellite imagery, used to describe spatial patterns of protective vegetation cover. This study proposes a method for ...

The Front Cover shows ammonia molecules adsorbed on an atomically flat platinum surface. An ultra-short laser pulse hits the surface and heats up metal electron rapidly to several thousand Kelvin within a few 100fs. This high electronic temperature causes fraction of desorb with hyperthermal velocity from More information can be found in Research Article by Kim Pappendorf et al..

A local to regional assessment of transported snow during snow storms or subsequent periods of strong winds is a prerequisite to reliably predict avalanche danger. Despite the fact that it has received continuing attention for decades, the problem of quantifying snow transport persists. Systems from point measurements to full three-dimensional simulations are in operation but all have their res...

Air conditioning is an important part in building services world-wide in both commercial and residential buildings. With the general attempt to reduce the usage of fossil fuels and increase the usage of green solutions, the market for solar air conditioning is growing rapidly. Solar air conditioning combines solar thermal absorbers to transfer solar radiation into heat with heat-driven cooling ...

Mangrove forests are one of the most productive and carbon dense ecosystems that are only found at tidally inundated coastal areas. Forest canopy height is an important measure for modeling carbon and biomass dynamics, as well as land cover change. By taking advantage of the flat terrain and dense canopy cover, the present study derived digital surface models (DSMs) using stereo-photogrammetric...